A retired U.S. general with deep knowledge of the geopolitical dynamics at play in Ukraine says the country’s military will stand its ground if Russian forces launch an assault.

“My experience was the Ukrainian infantry was very tough,” retired Army Lt. Gen. Mark Hertling, the former commander of U.S. Army Europe, told Military Times on Monday. “They are hard soldiers. They are used to hard conditions and their leadership was becoming more professional as we were working with them in Europe.”?

Russian troops have occupied Ukraine’s Crimea region since last week and appear poised to invade the eastern part of the country, which is heavily Russian. On Monday, the Russian navy’s Black Sea fleet commander gave Ukrainian forces in the Crimea an ultimatum to surrender by Tuesday morning or face an assault, according to Russia’s Interfax news agency, which cited a Ukrainian defense official. However, other reports indicate Russia has denied issuing such an ultimatum.

The Russian army relies heavily on tanks, but Hertling said the Ukrainian military’s ability to counter Russian armor is limited. Its forces have old T-72 tanks, BMP-1 fighting vehicles and third-generation anti-tank missiles.

It’s unclear what, if any, assistance the U.S. or NATO may provide Ukraine. For now, world leaders are scrambling to find a diplomatic solution. There are no NATO troops in Ukraine, and the North Atlantic Council has not been tasked with any military planning or response, said Col. Martin Downie, a NATO spokesman.

However, military relations between the U.S. and Ukraine go back several years. U.S. forces have trained extensively with the Ukrainian military, such as in the annual Rapid Trident exercise held in Ukraine; and Ukrainian noncommissioned officers have attended the U.S. Army’s Joint Multinational Training Command in Grafenwoehr, Germany, Hertling said.

Hertling was head of U.S. Army Europe from March 2011 until November 2012. During that time, the Ukrainians focused primarily on improving their skills for peacekeeping operations, but they’ve kept a wary eye to the east, he said.

Complicating matters, the Ukrainian military allows troops to be based close to their homes, so forces in eastern Ukraine are primarily Russian-speaking. That raises the possibility some could be sympathetic to Russia, Hertling said.

Already, the newly appointed head of Ukraine’s naval fleet defected to the Russians after assuming his position on Saturday, but his entreaties to fellow naval officers to join him have been rejected, according to media reports. In Crimea, where Russian troops have surrounded Ukrainian bases, a Russian commander appealed to Ukrainian troops in Simferopol to surrender. The Ukrainians refused.

The Russians may have better equipment, Hertling said, but if the Ukrainians are attacked, “they are going to fight.”